Scotish Antipathy Towards England
The key theme of the film is the antipathy felt by the Scots towards the English. It's fair to say that the two nations did not particularly care for each other and the Scots in particular wanted, and demanded, their independence from the English crown. This still continues today although the differences are fought at the ballot box rather than in hand to hand combat.
The Scots largely rejected English rule, disliking the monarch and feeling that they came off rather badly in most of the deals done between the two nations. However, there was also division when it came to the issue in that many Scottish noblemen wanted to remain under English rule because they felt that it was personally advantageous. They played both sides against the middle, which won them favors from the King, but also won them a target on their back when it came to William Wallace and his supporters.
The movie is based on historical fact so the theme also follows history in the way that this antipathy and hatred was exhibited.
Class Battles
Although it was rather soon in history for there to be an actual three tier class system with associated biases and rivalries, there is a definite divide in the film between noblemen and their "subjects." It was this divide that generally caused Scottish noblemen to err on the side of the English monarch. Noblemen believed that it would be more beneficial to them to side with the King, and generally did so. The only supporter Wallace had who was of noble stock was Robert the Bruce, and even he betrayed Wallace before seeing the error of his ways and becoming a genuine ally.
Love
Admittedly, this does not seem like much of a love story, but in fact it is, because the main motivation behind Wallace's hatred of the English was that they killed the love of his life. Even though he later became romantically involved with the French queen, he never really got over the loss of his wife, and never married again. The pain of this lost love was what generated his entire campaign against the English as he wanted above all else to avenge her death.
Skullduggery and Subterfuge
For the majority of the film, the noblemen whom Wallace believes to be with him are, in fact, against him. Robert Bruce pretends to be on Wallace's side, ready to back him up, only to betray him and give away his strategy and plans to the King, at his father's behest. After he does so, he regrets his actions, and begins to plot against his father alongside Wallace.
Similarly, when Wallace heads to a meeting with his noble "allies" he finds that he is arrested and handed over to the King as a result of their subterfuge and plotting behind his back.
